Treatment of waste

ABSTRACT

A method of destroying organic waste in a bath of molten metal and slag contained in a vessel (3) is disclosed. The method comprises injecting organic waste into the bath to form a primary reaction zone (13) in which the organic waste is thermally cracked and the products of the thermal cracking which are not absorbed into the bath are released into the space above the surface of the bath. The method further comprises injecting an oxygen-containing gas toward the surface of the bath to form a secondary reaction zone (17) in the space above the surface of the bath in which the oxidizable materials in the products from the primary reaction zone (13) are completely oxidized and the heat released by such oxidation is transferred to the bath. In order to facilitate efficient heat transfer from the second reaction zone (13) to the bath, the method further comprises injecting an inert or other suitable gas into the bath to cause molten metal and slag to be ejected upwardly from the bath into the secondary reaction zone (17).

The present invention relates to the destruction of organic waste and inparticular toxic organic waste.

The term "organic waste" is herein understood to include but is notlimited to toxic materials such as pesticides, herbicides, paints,medical residues, waste oils, contaminated solvents, and black liquor.

U.S. Pat. Nos. 4,574,714 and 4,602,574 by Bach and Nagel disclose amethod of destroying organic waste which is based on the use of a bathof molten metal, typically comprising at least 10% iron. The methodcomprises maintaining the bath at a temperature of at least 1400° C. andinjecting organic waste and oxygen-containing gas into the bath fromabove and/or below the surface of the bath to crack thermally (pyrolyse)and at least partially oxidize the oxidizable portions of the organicwaste. The amount of oxygen-containing gas added to the bath is selectedso that the stoichiometric ratio of oxygen to the oxidizable portion ofthe organic waste is at least 1:1 to ensure that there is sufficientoxygen input to oxidize completely the oxidizable portions in and abovethe surface of the bath.

The complete oxidation of the oxidizable portions of the organic wasteis particularly important to ensure that there are no hydrocarbonsremaining in the off-gas which are toxic or could form toxic organicmaterials such as furans and dioxins as the off-gas cools to ambienttemperature. A further important consideration in this regard is toensure that the concentration of oxygen in the off-gas is as low aspossible since there is some evidence that oxygen is favourable to theformation of at least dioxins.

A disadvantage of the method disclosed in the U.S. patents is that thereis a relatively high risk of the formation of a continuous path in thebath through which the organic waste can flow to escape the bath in asubstantially untreated form.

An object of the present invention is to provide a method of destroyingorganic waste which has a higher assurance against such break-out oforganic waste from the bath than is possible with the method disclosedin the U.S. patents.

According to the present invention there is provided a method ofdestroying organic waste in a bath of molten metal and slag contained ina vessel, the method comprising:

(a) injecting organic waste into the bath to form a primary reactionzone in which the organic waste is thermally cracked and the products ofthe thermal cracking which are not absorbed into the bath are releasedinto the space above the surface of the bath; and

(b) injecting an oxygen-containing gas towards the surface of the bathto form a secondary reaction zone in the space above the surface of thebath in which the oxidizable materials in the products from the primaryreaction zone are completely oxidized and the heat released by suchoxidation is transferred to the bath.

The above described method of the present invention provides a highlevel of assurance against unreacted or partially reacted organic wasteshort circuiting the process and reporting in the exit gas stream fromthe vessel. This is achieved by providing at least two separate reactionzones through which the organic waste must pass before exiting thevessel.

It is preferred that the method comprises injecting organic waste with acarrier gas into the bath to form the primary reaction zone. It ispreferred that the carrier gas be an inert gas.

It is preferred that the method further comprises injectingoxygen-containing gas into the primary reaction zone to at leastpartially oxidize the products of thermal cracking.

It is particularly preferred that the method comprises injecting theoxygen-containing gas into the primary reaction zone in a stoichiometricratio of oxygen to the oxidizable portion of at least 1:1.

It is preferred that the method further comprises controlling thetemperature in the secondary reaction zone to be at least 200° C. higherthan the temperature of the bath.

It is preferred particularly that the method comprises controlling thetemperature in the secondary reaction zone to be in the range of 1500°to 2700° C.

It is particularly preferred that the method further comprises injectingcarbonaceous material into the bath to form a carburizing zone in whichthe carbon in the carbonaceous material is dissolved into the bath.

The term "carbonaceous material" is herein understood to include: solidcarbonaceous fuels such as coke and coal; liquid fuels such as oil,light fuel oil, diesel oil and heavy fuel oil; and gaseous fuels, suchas natural gas, methane, ethane, propane, butane; or any mixtures of thefuels.

In the above described preferred embodiment the heat transferred to thebath from the secondary reaction zone is used to balance the heat lostfrom the bath in the endothermic reactions in the carburizing zone andin some instances in the primary reaction zone.

The principal purpose of oxygen injection into the primary reaction zoneis to at least partially oxidize the injected organic waste and theproducts of thermal cracking of the organic waste. However,concurrently, oxygen reacts with dissolved carbon in the bath to form COand with the bath metal to form metal oxides which either report to theslag or are reduced by dissolved carbon back to metal. The carburizationof the metal in the carburizing zone maintains the concentration ofdissolved carbon in the metal above an appropriate minimum level.

Typically, the products emerging from the primary reaction zone into thespace above the bath comprise the oxidizable materials CO, H₂ andcarbonaceous material arising from the thermal cracking of the organicwaste. The products may also contain volatilized species such as metalsand other compounds and elements which contaminate the organic waste orare added with the organic waste. Typically, the reaction products fromthe secondary reaction zone comprise CO₂ and H₂ O.

It is preferred that any volatilized species be scrubbed in thesecondary reaction zone.

It is preferred that the organic waste and oxygen-containing gas beinjected into the primary reaction zone through the bottom of thevessel.

It is preferred that the method comprises injecting a gas into the bathto cause molten metal and slag to be ejected upwardly from the bath intothe secondary reaction zone to facilitate efficient heat transfer to thebath and scrubbing of volatilized species and any particulate materialin the products from the primary reaction zone.

One particularly preferred embodiment comprises injecting the gas intothe carburizing zone of the bath with the carbonaceous material andinjecting the oxygen-containing gas towards the surface of the bathabove the carburizing zone so that the secondary reaction zone islocated immediately above the carburizing zone.

Another particularly preferred embodiment comprises injecting the gasinto the bath to form a gas injection zone adjacent the carburizing zoneand injecting the oxygen-containing gas into the space above the gasinjection zone.

It is preferred that the method comprises injecting into the primaryreaction zone an additive for forming preferred reaction products. Theadditive may be selected as required to match the composition of theorganic waste. By way of example, where the organic waste includeshalogens, such as chlorides, it is preferred that the additive comprisean alkaline earth for binding chemically with the halogens.

It is preferred that the method comprises injecting slag forming agentsinto the bath together with or separately to the organic waste tocondition the composition of the slag selectively. Typically, the slagforming agents comprise CaO and fluorspar.

It is preferred that the bath comprises at least 10% metal. It isparticularly preferred that the bath comprises at least 70% metal. It ismore particularly preferred that the bath comprises at least 80% metal.

It is preferred that the metal be selected from one or more of the groupcomprising iron, ferroalloys, tin, nickel, chromium, silicon, andcopper, and mixtures thereof. It is particularly preferred that themetal comprises iron.

It is preferred that the gas injected into bath to cause molten metaland slag to be ejected upwardly into the secondary reaction zone beselected from one or more of an inert gas, recycled process gas, naturalgas, carbon dioxide, propane, or butane, or mixtures of the gases. It isparticularly preferred that the inert gas be nitrogen.

It is preferred that the oxygen-containing gas be selected from thegroup comprising oxygen, air, oxygen enriched air, and steam. It isparticularly preferred that the oxygen-containing gas be air. It is moreparticularly preferred that the air be preheated. Typically, the air ispreheated to temperatures in the range of 900° to 1600° C.

The present invention is described further with reference to theaccompanying drawings in which:

FIG. 1 illustrates a preferred embodiment of a method of destroyingorganic waste in accordance with the present invention;

FIG. 2 illustrates another preferred embodiment of a method ofdestroying organic waste in accordance with the present invention; and

FIG. 3 illustrates a further preferred embodiment of a method ofdestroying organic waste in accordance with the present invention.

Each of the preferred embodiments of the method of the present inventionshown in FIGS. 1 to 3 is carried out in a vessel generally identified bythe numeral 3.

The vessel 3 may be of any suitable design of metallurgical vessel withrefractory lined internal walls and an outer metal shell. In thepreferred arrangement shown in FIGS. 1 to 3 the vessel 3 is a generallycylindrical shape disposed horizontally and has bottom tuyeres 5,7, aslag-metal tap 9, an air injection port 10, and an upper off-gas outlet11 at one end of the vessel 3. Typically, the ratio of the length andthe diameter of the vessel is 3:1.

The vessel 3 contains a volume of molten metal which comprises at least10% iron and a layer of slag (not shown) at a temperature of at least1400° C. The other metals in the bath may be selected as required and,by way of example, may also include one or more of ferroalloys, tin,nickel, silicon, and copper.

The preferred embodiment of the method shown in FIG. 1 comprisesinjecting organic waste and a suitable carrier gas such as an inert gasthrough the bottom tuyeres 5 to form a primary reaction zone indicatedschematically by the circle 13 which is located at the end of the vessel3 remote from the off-gas outlet 11. The organic waste is thermallycracked in the primary reaction zone 13 into C and H₂. A proportion ofthe products remain in the bath and the remainder of the products arereleased into the region in the space above the bath that is directlyabove the primary reaction zone 13.

The method shown in FIG. 1 also comprises injecting pre-heated air,typically at a temperature in the range of 900° to 1600° C., or anyother suitable oxygen-containing gas through injection port 10 towardsthe surface of the bath adjacent the primary reaction zone 13 to form asecondary reaction zone indicated schematically by the circle 17 in theregion of the space above the bath that is located between the regionthat is directly above the primary reaction zone 13 and the off-gasoutlet 11. Simultaneously, nitrogen or any other suitable gas isinjected through tuyeres 7 into the bath immediately below the secondaryreaction zone 17 and causes eruption of molten metal and slag insplashes and/or droplets from the surface of the bath into the secondaryreaction zone 17. Typically, the nitrogen is injected in an amountgreater than or equal to 0.1 Nm³ min⁻¹ tonne⁻¹ of molten metal in thebath. The pre-heated air completely oxidizes the products from theprimary reaction zone 13 as the products flow through the secondaryreaction zone 17 from the space directly above the primary reaction zone13 towards the off-gas outlet 11. Furthermore, the heat released by suchoxidation is efficiently transferred to the splashes and/or droplets ofmolten metal and slag and subsequently into the bath when the splashesand/or droplets fall downwardly to the surface of the bath. The splashesand/or droplets also scrub any volatilized species such as metalcontaminants and any particulate material in the organic waste as theproducts flow through the secondary reaction zone 17 and transfer thescrubbed values to the bath. It is noted that in effect the splashesand/or droplets of molten metal and slag form a curtain in the secondaryreaction zone 17 which is an effective and efficient means oftransferring heat to the bath and scrubbing volatilized species andparticulate material from products flowing through the secondaryreaction zone 17.

Typically, the temperature in the secondary reaction zone 17 iscontrolled to be at least 200° C. higher than that of the molten metal.Typically, the temperature in the secondary reaction zone 13 variesbetween 1500° C. and 2700° C.

It can be readily appreciated from the foregoing that the secondaryreaction zone 17 has three important functions. Specifically, thesecondary reaction zone 17:

(a) completely oxidizes any oxidizable portions in the products from theprimary reaction zone 13;

(b) ensures that the heat released by such oxidation is transferred tothe bath; and

(c) scrubs any volatilized species such as metal contaminants and anyparticulate material from the reaction products.

The preheated air may be injected into the secondary reaction zone 17 byany suitable means such as top-blowing single or multiple tuyeres orlances with one or more openings.

The preferred embodiment of the method of the invention shown in FIG. 2is similar to that shown in FIG. 1 and further comprises injectingoxygen or any other suitable oxygen-containing gas with organic wastethrough the tuyeres 5 into the primary reaction zone 13. The oxygen andorganic waste move upwardly towards the surface of the bath and arethermally cracked and at least partially oxidized. A proportion of theproducts of the thermal cracking and partial oxidation remain in thebath and the remainder of the products are released from the bath intothe space above the primary reaction zone 13. Typically, the productsreleased into the space above the primary reaction zone 13 comprise theoxidizable materials CO, H₂ and carbonaceous material arising from thethermal cracking of the organic waste, and other volatilized speciespresent as contaminants in the organic waste or added with the organicwaste.

It is noted that there are a number of possible reactions for oxygeninjected into the primary reaction zone 13 through the tuyeres 5. Inaddition to the series of preferred reactions noted in the precedingparagraph the oxygen can react with the iron and dissolved carbon in theiron to form FeO and CO. These reactions have the effect of decreasingthe amount of oxygen available for the preferred reactions on theproducts of thermal cracking of the organic waste and are undesirablefor this reason. In addition, the reactions are undesirable because theCO produced increases the gas volume and depletes the level of dissolvedcarbon in the iron and therefore decreases the capacity of the bath toreduce the FeO. These are essentially competing reactions in the sensethat varying the level of dissolved carbon in the iron has oppositeeffects on the reactions. By way of explanation, maintaining a highlevel of dissolved carbon in the iron results in a relatively high levelof CO and a low level of FeO and, on the other hand, maintaining a lowlevel of dissolved carbon in the iron results in a relatively low levelof CO and a relatively high level of FeO. As a consequence, it isnecessary to maintain the level of dissolved carbon in the iron withinan appropriate range.

By way of example, in situations where molten metal comprises at least60% iron the applicant has found that optimum operation can be achievedby maintaining the carbon concentration of the bath in the range of 1 to3 wt.% and the molten metal temperature between 1350° and 1600° C. Undersuch operating conditions it was found that substantially all the metaloxides injected into the bath were reduced to metals.

With the above in mind, the method shown in FIG. 2 comprises injectingcoal or any other suitable carbonaceous material into the bath throughthe bottom tuyeres 7 to form a carburization zone indicatedschematically by the arrow 15. The volatiles in the coal are thermallycracked and the carbon dissolves in the iron and disperses through thebath and in particular into the primary reaction zone 13.

It is noted that the heat transfer from the secondary reaction zone 17to the bath is important since the reactions in the carburization zone15 are essentially endothermic and it is important to balance the heatloss due to such reactions to maintain the temperature of the bath at alevel which can thermally crack the organic waste.

The preferred embodiment of the method of the present invention shown inFIG. 3 is similar to that shown in FIGS. 1 and 2 in that it includesdirecting the organic waste through a primary reaction zone 13 and asecondary reaction zone 17.

The method also includes injecting one or more additives into the moltenmetal through a bottom tuyere 61. The additives are selected to convertparticular components of the organic waste, such as halogens, into moreinert and/or more readily disposable forms.

The method also includes subsequently directing the products emergingfrom secondary reaction zone 17 through a tertiary reaction zoneindicated schematically by the numeral 23 to provide a higher level ofassurance. The tertiary reaction zone 23 is formed by injectingoxygen-containing gas through injection port 10 towards the surface ofthe bath and nitrogen or any other suitable gas through tuyeres into thebath below the tertiary reaction zone 23 to cause eruption of moltenmetal and slag in splashes and/or droplets into the tertiary reactionzone 23.

Typically, the oxygen-containing gas injected into the secondaryreaction zone 17 comprises oxygen-enriched air containing 25% O₂preheated to a temperature of 1350° C. and the oxygen-containing gasinjected into the tertiary reaction zone comprises preheated air.

Many modifications may be made to the preferred embodiments of themethod of the present invention shown in FIGS. 1 and 2 without departingfrom the spirit and scope of the present invention.

In this regard, whilst in the preferred embodiments the organic waste,oxygen-containing gas, and carbonaceous material are injected into thebath to form separate essentially macro-sized reaction and carburizationzones in the bath, it can readily be appreciated that the presentinvention is not so limited and the injection of the constituents intothe bath can be controlled to form arrays of separate essentiallymicro-sized primary reaction and carburization zones.

Furthermore, whilst the preferred embodiment shown in FIG. 2 includesthe location of the secondary reaction zone 17 immediately above thecarburization zone 15, it can readily be appreciated that the presentinvention is not so limited and the secondary reaction zone 17 may belocated above a section of the bath that is adjacent to thecarburization zone 15.

Furthermore, whilst in the preferred embodiments shown in FIGS. 2 and 3the carbonaceous material is injected into the bath through bottomtuyeres, it can readily be appreciated that the present invention is notso limited and extends to the use of any suitable means includingtop-blowing tuyeres and lances.

We claim:
 1. A method of destroying organic waste in a bath of moltenmetal and slag contained in a vessel, the method comprising:(a)injecting organic waste into the bath to form a primary reaction zone inwhich the organic waste is thermally cracked and the products of thethermal cracking which are not absorbed into the bath are released intothe space above the surface of the bath; (b) injecting anoxygen-containing gas towards the surface of the bath to form asecondary reaction zone in the space above the surface of the bath inwhich the oxidizable materials in the products from the primary reactionzone are completely oxidized and the heat released by such oxidizationis transferred to the bath; and (c) injecting a gas into the bath tocause molten metal and slag to be ejected upwardly from the bath intosaid secondary reaction zone to facilitate efficient heat transfer tothe bath and scrubbing of volatilized species in the products from theprimary reaction zone.
 2. The method defined in claim 1, comprisinginjecting organic waste and a carrier gas into the bath to form theprimary reaction zone.
 3. The method defined in claim 1 or claim 2,further comprising injecting oxygen-containing gas into the primaryreaction zone to at least partially oxidize the products of thermalcracking.
 4. The method defined in claim 3, comprising injecting theoxygen-containing gas into the primary reaction zone in a stoichiometricratio of oxygen to the oxidizable portion of at least 1:1.
 5. The methoddefined in any one of the preceding claims, comprising controlling thetemperature in the secondary reaction zone to be at least 200° C. higherthan the temperature of the bath.
 6. The method defined in claim 5,comprising controlling the temperature in the secondary reaction zone tobe in the range of 1500° to 2700° C.
 7. The method defined in claim 6,further comprising injecting carbonaceous material into the bath to forma carburizing zone in which the carbon in the carbonaceous material isdissolved into the bath.
 8. The method defined in claim 7, comprisinginjecting the organic waste into the primary reaction zone through thebottom of the vessel.
 9. The method defined in claim 4, comprisinginjecting the oxygen-containing gas into the primary reaction zonethrough the bottom of the vessel.
 10. The method defined in claim 1,comprising injecting the gas into the carburizing zone of the bath withthe carbonaceous material and injecting the oxygen-containing gastowards the surface of the bath above the carburizing zone so that thesecondary reaction zone is located immediately above the carburizingzone.
 11. The method defined in claim 1, comprising injecting the gasinto the bath to form a gas injection zone adjacent the carburizing zoneand injecting the oxygen-containing gas into the space above the gasinjection zone.
 12. The method defined in claim 1, further comprisinginjecting into the primary reaction zone an additive for formingpreferred reaction products
 13. The method defined in claim 12, whereinthe additive comprises an alkaline earth.
 14. The method defined inclaim 13 further comprising, injecting slag forming agents into the bathtogether with or separately from the organic waste to conditionselectively the composition of the slag.
 15. The method defined in claim14, wherein the slag forming agents comprise CaO and fluorspar.
 16. Themethod defined in claim 15, wherein the bath comprises at least 10%metal.
 17. The method defined in claim 16, wherein the bath comprises atleast 70% metal.
 18. The method defined in claim 17, wherein the bathcomprises at least 80% metal.
 19. The method defined in claim 18,wherein the metal is selected from one or more of the group comprisingiron, ferroalloys, tin, nickel, chromium, silicon, and copper, andmixtures thereof.
 20. The method defined in claim 19, wherein the metalcomprises iron.
 21. The method defined in claim 1, wherein the gasinjected into bath to cause molten metal and slag droplets to be ejectedupwardly into the secondary reaction zone is selected from one or moreof an inert gas, a recycled process gas, natural gas, carbon dioxide,propane, or butane, or mixtures of the gases.
 22. The method defined inclaim 21, wherein the inert gas is nitrogen.
 23. The method defined inclaim 22, wherein the oxygen-containing gas injected into the secondaryreaction zone is selected from one or more of the group comprisingoxygen, air, oxygen enriched air, and steam.
 24. The method defined inclaim 23, further comprising preheating the oxygen-containing gas to atemperature in the range of 900° to 1600° C.